Femto base stations, also called home base stations, have attracted much interest recently in the wireless industry. The standardization process for home base stations is ongoing in 3GPP for Universal Terrestrial Radio Access (UTRA), Evolved UMTS Radio Access (E-UTRA) and Worldwide Interoperability for Microwave Access (WiMAX). Furthermore, in both UTRA and E-UTRA, advanced features related to home base stations such as mobility procedures, interference management and control etc are also being introduced. Home base stations are already operational in other technologies such as Global System for Mobile communication (GSM) and 3GPP2 CDMA technologies (e.g. CDMA2000 1×RTT and High Rate Packet Data, HRPD).
In legacy UTRAN specifications, four classes of base stations (BS) are defined, namely the wide area BS that serves macro cell deployment, the medium range BS that serves micro cell deployment, the local area BS that serves pico cell deployment and the home base station serving the private localized premises, like a home or an office. In UTRAN, the home BS is also called a home NodeB (HNB). In E-UTRAN specifications three classes of base stations are defined; Wide area BS, local area BS and home base station. In E-UTRAN the home BS is also called a Home eNodeB (HeNB).
A home base station may also be referred to as a home access point, femto base station, femto access point, home NodeB, or home eNodeB. Some particular examples of Home Base Stations are UTRAN FDD/TDD home NodeB, E-UTRAN TDD/FDD home eNB (eNodeB), GSM home base station, CDMA2000 1×home BS, HRPD home BS, or WiMAX home base station. For simplicity and consistency, we will use the term home base station (HBS) in the rest of the disclosure. This term is intended to covers all types of home access points, including but not limited to those mentioned above. It should be understood that a home base station may not only deployed in a private residence, but also in other public or private premises such as shopping malls, office buildings, etc.
Depending on the operator, a Home Base Station may share the carrier with macro, micro or pico base stations, i.e. non home base stations. This may be referred to as a mixed carrier scenario. Alternatively, the HBS may be assigned a carrier which is used only for the operation of home base stations, i.e. a dedicated carrier scenario.
One main difference between a home base station and other base station classes is that a Home Base Station is assumed to be owned by a private subscriber, who has the liberty to install it at any location. Thus, strict network planning is not possible in case of Home Base Station deployment. This is in contrast to other base station classes, which are deployed by an operator according to well-defined principles. The lack of precise network planning of Home Base Stations and their dense deployment may have the following consequences:                High interference towards other base stations, including other home BSs and the surrounding network, e.g. macro base stations.        In case of dense Home BS deployment, the UE may detect, measure and report a large number of cells, which are served by home base stations but are not required for mobility.        
Access to a home base station may be under the control of the operator or the owner of the home base station. The access control mechanism for a home BS decides if a given user may or may not connect to that home base station. The selection of the access control mechanism has a large impact on the performance of the overall network, mainly due to its role in the definition of interference. In UTRAN and E-UTRAN, the concept of a Closed Subscriber Group (CSG) exists. According to the CSG concept, only a subset of users, defined by the owner of the home base station, are allowed to connect to that particular home base station. Because access to a HBS may be restricted only to certain users, appropriate mobility procedures to prevent unnecessary handovers towards the non-allowed home base stations are specified for UTRAN as well as for E-UTRAN. This implies that before initiating a handover to a neighbor cell, the serving network node may need to know whether the target cell is a CSG cell or not, i.e. whether the target cell is served by a HBS using the CSG concept.
Currently, network deployments with several layers comprising macro base stations, pico base stations, home base stations etc are gaining popularity. In certain areas, coverage from macro layer deployment overlaps with areas covered by micro, pico or femto network deployments. Such a network or deployment is called a heterogeneous network. These heterogeneous network scenarios are expected to become more and more popular as a direct consequence of the proliferation of pico, femto, and home eNBs. In such heterogeneous network deployments, mobility management is becoming an even more challenging task, because it is quite probable that Physical Cell Identities, PCIs, are frequently reused. Hence, a serving node in many cases may have to know whether the target cell belongs to a macro, pico or home base station etc.
Thus, in a scenario with home base stations, as well as in heterogeneous networks, it is beneficial to uniquely determine the identity and/or type of potential target cells. This requires information which is transmitted in the cell's system information. Hence, the user equipment (UE) is expected to acquire system information from surrounding base stations, such as neighboring HBSs, and report this information to the network. One example of system information that the UE may need to acquire is the Cell Global Identifier (CGI), which is a unique identifier of a cell. The CGI or E-UTRAN CGI (ECGI) acquisition is typically performed by the UE in response to an explicit request received from the serving network node. One example scenario where this may occur is when the UE performs neighbor cell measurements on potential target cells for handover, i.e. to support mobility.
As part of the neighbor cell measurement procedure, the UE will send a measurement report containing neighbor cell measurements such as Reference Symbol Received Power (RSRP) and/or Reference Symbol Received Quality (RSRQ) in E-UTRAN, or Common Pilot Channel Received Signal Code Power (CPICH RSCP) and/or Common Pilot channel received energy per chip divided by power spectral density (CPICH Ec/No) in UTRAN. The serving network node typically uses these measurements to determine if the UE would be better served by one of the neighbor cells, i.e. whether to initiate a handover (HO).
The measurement report also comprises the physical cell identity (PCI) of the target cell to the serving network node, e.g. the serving eNodeB in E-UTRAN. The PCI is an identity which identifies the target cell, but it is typically not unique within the network. In current E-UTRAN specifications, for instance, there are only 504 different PCIs defined. This is because the PCI is broadcast at frequent intervals in the cell, so its length is restricted to only a few bytes in order to consume less radio resources. As a consequence, in a large network the same PCI may need to be reused in several cells and is therefore not guaranteed to be unique, or even to uniquely identify the type of the cell. In a dense Home Base Station deployment scenario, the PCIs are more frequently reused, due to the large number of cells and smaller cell sizes.
Therefore, in a situation where the serving network node is not able to derive the necessary information from the PCI, the serving network node may also request the UE to decode and report the Cell Global Identifier (CGI) of the target cell. For example, based on the reported PCI, the network node may suspect that the target cell belongs to a CSG, a Home Base Station or to any similar node as part of the heterogeneous network. In order to prevent a HO command to a non-allowed Home Base Station, e.g. a CSG cell to which the UE does not have access, the serving network node needs to be able to uniquely identify the cell, or at least determine with certainty whether the cell is served by a HBS or not, and in particular whether it is associated with a CSG. However, since the PCI is not unique, the network node cannot establish this based on the PCI alone. The CGI, however, is an identity which is unique in the network, thereby allowing the network to distinguish between macro BSs and home BSs, or to uniquely identify that the reported cell is associated with a CSG. Hence, to confirm its hypothesis that the target cell is served by a HBS, the network may request the UE to decode and report the target cell's CGI or ECGI.
The procedure and the associated requirements for the UE reporting of the target cell's CGI or ECGI are specified in both UTRAN and E-UTRAN. One key aspect of the CGI decoding is that it is performed by the UE during autonomous gaps, which are created by the UE itself. During the autonomous gaps, the UE interrupts its reception and transmission of data in the serving cell. The reason for acquiring the target cell CGI during autonomous gaps is that the typical UE implementation is not capable of simultaneously receiving data from the serving cell and acquiring the target cell's system information, which contains the CGI. Furthermore, the CGI acquisition of an inter-frequency or inter-Radio Access Technology (inter-RAT) target cell requires the UE to switch carrier frequency, which means it cannot communicate with the serving cell at the same time. Hence, the use of autonomous gaps is necessary for acquiring the target cell's CGI. These autonomous gaps are also interchangeably referred to as measurement occasions, because the gaps are the occasions during which the UE measures the CGI of the target cell.
The CGI is sent over a system information block (SIB). In E-UTRAN, the CGI is called E-UTRAN CGI (ECGI), and is transmitted in system information block type 1 (SIB1). However, the acquisition of ECGI requires the UE to first read the master information block (MIB) of the target cell, which is transmitted on the physical broadcast channel (PBCH) with a periodicity of 40 ms. Within the 40 ms period, the PBCH is repeated in every frame. For example, in E-UTRAN, the length of an E-UTRAN frame is 10 ms, and the PBCH will be repeated in every fourth frame. The MIB enables the UE to acquire information such as system frame number (SFN), cell transmission bandwidth etc. Hence, after acquiring the MIB, the UE reads the system information block type 1 (SIB1), which contains the ECGI and is transmitted with a periodicity of 80 ms on DL-SCH. The home eNodeB can be deployed on a shared carrier or on a dedicated carrier as described earlier. Therefore EGCI requirements in E-UTRAN are specified for the following two scenarios:                Intra-frequency ECGI reporting        Inter-frequency ECGI reporting        
The UE is required to report the intra-frequency ECGI within about 200 ms, including processing time of the measurement request, after receiving a request from the serving network node, for a target intra-frequency cell provided that the target cell's SINR experienced by the UE is at least −6 dB or higher, and to report an inter-frequency ECGI within about 200 ms, including processing time of the measurement request, provided that the target cell's SINR is at least −4 dB or higher. During the acquisition of the target cell's ECGI on the serving carrier frequency the UE is allowed to create autonomous gaps in the downlink. Those gaps result in interruptions in the UE downlink reception from the serving node, and uplink transmission to the serving node. That is to say, the UE cannot receive signals from or transmit signals to its serving node during an autonomous gap. The duration of the autonomous gap may vary depending upon the UE implementation, but typically comprises 3 sub-frames.
In UTRAN, the CGI is transmitted in the system information block type 3 (SIB3). But in order to read the SIB3 the UE has to first read the MIB. Hence the UE can determine the CGI of a neighbor cell by reading its MIB and system information block type 3 (SIB3), which are sent on the broadcast channel (BCH). The MIB is transmitted every 20 ms. The SIB3 may have different periodicity, which is configured by the network. As compared to E-UTRAN, in UTRAN the target cell's CGI acquisition time is typically much longer, e.g. more than 1 second depending upon the periodicity of the SIB3. Furthermore, due to the autonomous gaps created by the UE to acquire the target cell's CGI, the interruption of the data transmission and reception from the serving cell can be 600 ms or longer.
The concepts of autonomous gaps and CGI/ECGI acquisition are also relevant for self organizing networks (SON). The SON function in E-UTRAN and UTRAN allows the operator to automatically plan and tune the network parameters and network nodes. The conventional method is based on manual tuning, which consumes enormous amounts of time and resources, and requires considerable involvement of work force. Due to network complexity, a large number of system parameters, Inter-Radio Access Technologies (IRAT) etc., it is very attractive to have reliable schemes to perform the test of self organization in the network whenever necessary.
An operator may also add or delete a cell or an entire base station, which may serve multiple cells. Especially new cells are added more frequently during an early phase of network deployment. In the later stages, an operator can still upgrade the network by adding more carriers or more base stations on the same carrier. It can also add cells related to another technology. The network may automatically detect the new cells and their relationship to existing cells in a process referred to as automatic neighbor cell relation (ANR) establishment, which it is part of the self organizing network (SON) functionality. In order to ensure correct establishment of the neighbor cell relation, the serving cell requests the UE to report the CGI of the new target cell, whose PCI is identified and reported to the said serving cell. The CGI acquisition requires the UE to read the target cell's system information and is thus carried out by the UE during autonomous gaps. As in the case of home inbound mobility, the CGI acquisition for ANR purposes also leads to interruption of data from the serving cell.
As explained above, the UE autonomously creates measurement gaps for acquiring the system information of the target cell to decode its CGI in home BS inbound mobility scenarios, for ANR purposes, or in any similar scenario. In other words, the length (L) of the measurement gaps and number (N) of measurement gaps depend upon the UE itself. No signaling or any related information about the autonomous gaps is exchanged between the UE and the serving network node. Since the serving network node is unaware of the exact occasions and number of autonomous gaps created by the UE, it does not know when the UE can be conveniently scheduled without losing data due to the gaps.
One solution to this problem is to not allow the serving network node to schedule the UE during the entire time while the UE is acquiring system information, e.g. decoding the CGI, of a target cell. As mentioned above, this period is about 200 ms in E-UTRAN and may be over one second in UTRAN. Thus, this solution leads to a long interruption in the transmission/reception between the serving cell and the UE while the target cell's CGI decoding is performed. This may cause significant degradation of real time services such as voice.
Another prior art solution is to use an aggressive approach, i.e. to continue scheduling the UE during the period when the UE is acquiring system information from other cells, without any regard for the autonomous gaps. A drawback of this approach is that if the UE receives an uplink grant which coincides with an autonomous gap, the UE will not be able to use the grant. Thus, the associated uplink resources, which could potentially have been assigned to another UE, will be wasted. If a downlink transmission to the UE coincides with an autonomous gap, the UE will not receive the information and retransmission will be required.
The performance degradation resulting from these drawbacks may be significant, in particular in a dense home base station scenario or in heterogeneous networks, where there are many potential target cells for the UE to measure on. It is therefore important to minimize the duration of the data interruption from the serving cell while the UE is acquiring system information, in particular CGI or ECGI.
There is thus a need in the art for improving the performance of wireless communication systems, in particular in a scenario when UEs may need to acquire system information from neighboring cells.